A spread-spectrum (SS) communication system using a wide frequency band involves a problem that high level narrow band interference often disables communication or increases errors. In order to overcome the problem, a filter using a surface acoustic wave (SAW) has been invented.
The present inventors et al. have already filed Japanese patent applications No. 180822/1988, 278769/1988 and 284884/1988 (all corresponding to U.S. Pat. No. 5,028,101) in the attempt to overcome the above-identified problem.
There is room for improvements in the following respects in the preceding inventions.
In the applications 278769/1988 and 284884/1988, for example, adaptive operation for narrow band interference suppression is performed using a self-biasing effect of pn diode arrays provided in a surface-acousticwave (SAW) element. In this case, the input signal needs to have a relatively large power.
In the application 180822/1988, since it uses a programmable notch filter which has not an adaptive function, an entered signal monitor circuit and a bias control circuit are required as external circuits.